Keep unused annot on params

Author: som-snytt

compiler/src/dotty/tools/dotc/transform/PostTyper.scala

@@ -247,8 +247,10 @@ class PostTyper extends MacroTransform with InfoTransformer { thisPhase =>
             if sym.is(Param) then
               registerIfUnrolledParam(sym)
               // @unused is getter/setter but we want it on ordinary method params
-              if !sym.owner.is(Method) || sym.owner.isConstructor then
-                sym.keepAnnotationsCarrying(thisPhase, Set(defn.ParamMetaAnnot), orNoneOf = defn.NonBeanMetaAnnots)
+              // @param should be consulted only for fields
+              val unusing = sym.getAnnotation(defn.UnusedAnnot)
+              sym.keepAnnotationsCarrying(thisPhase, Set(defn.ParamMetaAnnot), orNoneOf = defn.NonBeanMetaAnnots)
+              unusing.foreach(sym.addAnnotation)
             else if sym.is(ParamAccessor) then
               // @publicInBinary is not a meta-annotation and therefore not kept by `keepAnnotationsCarrying`
               val publicInBinaryAnnotOpt = sym.getAnnotation(defn.PublicInBinaryAnnot)

tests/warn/i23033.scala

@@ -0,0 +1,13 @@
+//> using options -Werror -Wunused:all
+
+import scala.annotation.unused
+import scala.concurrent.ExecutionContext
+import scala.util.NotGiven
+
+object Test {
+  given [T](using @unused ev: NotGiven[T <:< Int]): AnyRef with {}
+}
+object Useful:
+  given [T](using @unused ec: ExecutionContext): AnyRef with {}
+object Syntax:
+  given [T] => (@unused ec: ExecutionContext) => AnyRef